1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus for recording and/or reproducing information on and/or from an information recording medium such as a magnetic disk or a magneto-optical disk, and more particularly to a velocity control unit for a head in a seek mode which is used to record and/or reproduce information.
2. Related Background Art
When an information recording/reproducing head is to be driven to a desired position (track) on an information recording medium, velocity of the head is controlled. In a prior art velocity control system, the velocity of the head is sequentially monitored so that the head seeks in accordance with a predetermined drive scheme. FIG. 1 shows relationships among a reference velocity, an actual velocity and current supplied to a head drive actuator. The reference velocity V.sub.ref represents a drive schedule velocity of the head and it is calculated in accordance with a remaining distance to a target point. The reference velocity V.sub.ref is given by: EQU V.sub.ref =[2.multidot..alpha.(S-.lambda./2.multidot.N)].sup.1/2( 1)
where S is the target moving distance, .alpha. is a deceleration amount and N is a zero-crossing count. The actual velocity of the head is sequentially detected so that the head velocity follows the reference velocity. For example, zero-crossing points of a tracking error signal are detected to count a time period .DELTA.t.sub.n between a zero-crossing point and the next zero-crossing point, and a current velocity V.sub.n of the head is calculated based on the time .DELTA.t.sub.n and a track pitch .lambda.. It is represented by a formula as follows. A distance between the zero-crossing points corresponds to 1/2 of the track pitch .lambda.. EQU V.sub.n =.lambda./2.times.1/.DELTA.t.sub.n ( 2)
When the head velocity is to be controlled, a command value for an actuator is calculated at every predetermined period based on the current velocity and the target velocity, and the head velocity is controlled based on the calculated command value. The command value A.sub.ct is calculated as follows: EQU A.sub.ct =K(V.sub.ref -V.sub.n) (3)
where K is a feedback gain of a velocity control system. In this manner, in the prior art, the head velocity is sequentially detected and the command value calculated based on the detected head velocity and the target velocity is fed back at every predetermined period so that the head is driven to follow the target velocity as shown in FIG. 1 and the head is seek controlled to the target position in accordance with a predetermined drive schedule.
However, the prior art velocity control system has a problem in that the detected current head velocity does not always match a real velocity, but it is delayed from the actual velocity. The delay of the detected velocity is now described in detail with reference to FIGS. 2A and 2B. FIG. 2A shows a head reference velocity (target velocity), a real velocity and a detected current velocity, and FIG. 2B shows a tracking error signal. When the head velocity is to be detected, it is calculated based on the distance between the zero-crossing points and the time to move thereacross so that the calculated velocity is an average velocity between the zero-crossing points. As shown in FIGS. 2A and 2B, the velocity calculated at each detection of the zero-crossing point is an average between the zero-crossing points as shown by a stepwise velocity curve. At the time of detection of the zero-crossing point at which the velocity is actually calculated, a delayed velocity from a real velocity is calculated as a current velocity. This trend is remarkable in a low velocity area where the time required to pass the distance between the zero-crossing points is long. As a result, in the prior art, the velocity control is unstable and exact seeking operation of the head is difficult to attain.